xl2tpd is an implementation of the Layer 2 Tunnelling Protocol (RFC 2661),
which works by opening a pseudo-tty for communicating with pppd.

L2TP allows you to tunnel PPP over UDP. Some ISPs use L2TP to tunnel user
sessions from dial-in servers (modem banks, ADSL DSLAMs) to back-end PPP
servers. Another important application is Virtual Private Networks where
the IPsec protocol is used to secure the L2TP connection (L2TP/IPsec,
RFC 3193).
